Wire-drawing method and system

ABSTRACT

A wire-drawing method comprises providing a rod comprising a wrapped sheet, wherein the sheet comprises a plurality of copper layers and a plurality of graphene layers; extracting an inner layer of the wrapped sheet from the rod to form a spiral; and forming a wire by feeding the spiral through an opening of a die unit.

TECHNICAL FIELD

The disclosure relates to a method and system for wire-drawing a wrappedsheet comprising a plurality of copper layers and a plurality ofgraphene layers, in particular to form a wire of enhanced electricconductivity with tailored dimensions.

BACKGROUND

Copper is known as a cost-effective and reliable electrically conductivematerial, second only to silver in its ability to conduct electricity.Increasing demand for electrical power and the need to generate andtransmit electricity more efficiently, and more environmentallyfriendly, has spurred the search for conductors with an increasedelectric conductivity.

Intensive investigations have been made for improving the electricalperformance of copper nanowires compared to copper thin films, such asdescribed in T. Gao et al., Journal of Applied Physics 2013, 114,063107. However, the effective conductivity of these nanowires isusually not significantly larger than its ideal bulk value, and may behighly dependent on technological constraints.

Co-pending U.S. application Ser. No. 15/290,865 describes techniques forincreasing the electric conductivity in a composite structure comprisingfirst and second graphene layers sandwiching a copper layer.

BRIEF SUMMARY OF THE DISCLOSURE

Known techniques for producing conductive elements may be enhanced byforming multilayer composite structures comprising wires with aplurality of copper layers and a plurality of graphene layers.

These objectives are achieved with a wire-drawing method according toindependent claim 1 and a wire-drawing system according to independentclaim 16. The dependent claims relate to preferred embodiments.

The disclosure relates to a wire-drawing method, comprising: providing arod comprising a wrapped sheet, wherein the sheet comprises a pluralityof copper layers and a plurality of graphene layers; extracting an innerlayer of the wrapped sheet from the rod to form a spiral; and forming awire by feeding the spiral through an opening of a die unit.

The wire-drawing technique allows to efficiently form a wire of adesired shape and diameter from a wrapped multilayer sheet, inparticular by controlling a shape and/or diameter of the opening of thedie unit.

Extracting an inner layer of the wrapped sheet may comprise pushingand/or pulling the wrapped sheet through the opening of the die unit.

In an example, the opening has an elliptic cross section, in particulara circular cross section.

The spiral may be fed through the opening in a feeding direction.

In an example, the opening has an inclination angle with respect to thefeeding direction of the spiral.

Feeding the spiral through an opening having an inclination angle allowsto twist the layers of the spiral with respect to one another, which mayassist in tuning the mechanical and/or electrical properties of theresulting wire.

In an example, the inclination angle is no smaller than 5°, inparticular no smaller than 10° or 15°.

In an example, the inclination angle is no larger than 30°, inparticular no larger than 25° or 20°.

Forming the wire may comprise feeding the spiral through a first openingof the die unit, and subsequently feeding the spiral through a secondopening of the die unit downstream of the first opening, wherein thesecond opening is smaller than the first opening.

A multi-stage drawing through openings with increasingly smallerdimensions may allow a careful tailoring of a size and diameter of theresulting wire.

In an example, a diameter and/or a surface area of the second opening issmaller than the diameter and/or surface area of the first opening.

In an example, the first opening and/or the second opening has anelliptic cross section, in particular a circular cross section.

The first opening may have a first inclination angle with respect to afeeding direction of the spiral, and/or the second opening may have asecond inclination angle with respect to a feeding direction of thespiral.

In an example, the first inclination angle and/or the second inclinationangle is no smaller than 5°, in particular no smaller than 10° or 15°.

In an example, the first inclination angle and/or the second inclinationangle is no larger than 30°, in particular no larger than 25° or 20°.

In some examples, the second inclination angle is equal to the firstinclination angle.

In other examples, the second inclination angle differs from the firstinclination angle.

Varying the inclination angle allows to fine-tune the twist angle, andhence the mechanical and/or electrical properties of the resulting wire.

The spiral may be fed through one or more additional openings to furtherfine-tune the mechanical and/or electrical properties of the wire.

In particular, forming the wire may comprise feeding the spiral througha third opening of the die unit downstream of the second opening,wherein the third opening is smaller than the second opening.

In particular, a diameter and/or a surface area of the third opening maybe smaller than a diameter and/or a surface area of the second opening.

The third opening may have an elliptic cross section, in particular acircular cross section.

The third opening may have a third inclination angle with respect to afeeding direction of the spiral.

The third inclination angle may be no smaller than 5°, in particular nosmaller than 10° or 150° The third inclination angle may be no largerthan 30°, in particular no larger than 25° or 20°.

In some examples, the third inclination angle is equal to the secondinclination angle and/or the first inclination angle.

In other examples, the third inclination angle differs from the secondinclination angle and/or the first inclination angle.

In an example, the feeding the spiral comprises pushing and/or pullingthe spiral through the opening.

Pushing and/or pulling the spiral through the opening may simultaneouslyprovide the necessary pulling and fusing forces to form a multilayercomposite wire structure.

In an example, the feeding the spiral comprises pulling the spiralthrough the opening by means of a plurality of pulling rolls, inparticular a plurality of rotating pulling rolls.

In an example, the pulling rolls may in addition orbit around thespiral, in particular around a longitudinal axis of the spiral.

Orbiting the pulling rolls around the spiral with a pre-selectablespacing between them may allow to provide a wire with a desired andcarefully controlled outer diameter and optimum round shape.

A copper layer, in the sense of the present disclosure, maybe understoodto denote a layer comprising copper. In some examples, the copper layeris a pure copper layer, in particular a copper layer comprising at least90% copper by weight. In other examples, the copper layer may comprisecopper and other components and materials in addition to the copper.

In an example, at least part of at least one copper layer of theplurality of the copper layers has a Cu (111) crystallographicorientation, which, as used herein, refers to the lattice plane ofCopper having 1, 1, and 1, as its Miller Indices.

The sheet according to the disclosure may comprise any number of copperlayers.

According to an example, at least one copper layer of the plurality ofcopper layers may have a thickness of no more than 500 μm, and inparticular no more than 100 μm or no more than 50 μm.

In an example, at least one copper layer of the plurality of copperlayers has a thickness of no larger than 25 μm or no larger than 10 μm,and in particular no larger than 5 μm.

Thin copper layers reduce the volume percentage of copper in favor ofgraphene in the resulting multi-composite wire structure, which mayenhance the electric conductivity, thermal conductivity and/ormechanical strength of the resulting wires.

In addition, reducing the thickness of the copper layers may reduce theamount of inelastic scattering of electrons within the copper, which mayfurther enhance the electric conductivity.

In an example, at least one copper layer of the plurality of copperlayers has a thickness of no larger than 2 μm, and in particular nolarger than 1 μm.

In an example, at least one copper layer of the plurality of copperlayers has a grain size of at least 1 μm, and in particular at least 5μm.

Enhancing the grain size of the copper layer may further reduceinelastic scattering of electrons within the copper layer, whichenhances the electric conductivity.

In an example, at least one copper layer of the plurality of copperlayers has a grain size of at least 10 μm, and in particular at least 50μm.

Grain size, in the sense of the present disclosure, may refer to anaverage distance between grain boundaries.

In an example, the sheet comprises a first copper layer on a firstsurface side of the sheet, and a second copper layer on a second surfaceside of the sheet opposite from the first surface side.

In this configuration, the windings of the spiral comprise copper on anouter surface layer, which prevents the windings from fusing togetherhomogeneously and provides optimum conditions for connecting theresulting wire to sockets, plugs, etc., such as by means of soldering,clamping, crimping, etc.

Graphene layers may be provided in the interior of the sheet to enableeasy sliding of the copper layers, which may assist in extracting theinner layer of the wrapped sheet and forming the spiral.

In an example, the sheet comprises a copper layer and first and secondgraphene layers sandwiching the copper layer.

In general, the sheet may comprise any number of graphene layers.

At least part of the graphene layers may alternate with at least part ofthe copper layers in the sheet.

The resulting multilayer composite structure of copper and graphene maydemonstrate enhanced electrical conductivity, thermal conductivity andmechanical strength.

Graphene, in the sense of the present disclosure, may refer to acarbon-comprising layer in which the carbon atoms are, as in graphite,hexagonally arranged and generally sp² hybridized. The resultingstructure may be a honeycomb-shaped hexagonal pattern or fusedsix-member carbon rings.

The term graphene may refer to a configuration with a carbon monolayer,but in practice a small number of layers may also be denoted asgraphene.

In some examples, the graphene comprises only carbon. In other examples,the graphene may be doped to comprise atoms and/or molecules differentfrom carbon atoms.

In an example, at least one graphene layer of the plurality of graphenelayers has a thickness of no larger than 5 nm, and in particular nolarger than 2 nm.

In an example, at least one graphene layer of the plurality of graphenelayers may comprise at most 20 graphene monolayers, and in particular atmost 10 graphene monolayers.

In particular, at least one graphene layer of the plurality of graphenelayers may be a monolayer or a bi-layer.

In an example, at least one graphene layer of the plurality of graphenelayers comprises metal atoms, in particular metal atoms ring-centered ongraphene rings.

A functionalization of graphene with ring-centered delocalized metalatoms may create anchoring points for firmly connecting the graphenelayers to the copper matrix without changing the hexagonal latticeproperties of the graphene structure.

In an example, the metal atoms comprise silver atoms.

The disclosure further relates to a wire-drawing system, comprising: adie unit comprising an opening; an extracting unit to extract an innerlayer of a wrapped sheet to form a spiral, wherein the sheet comprises aplurality of copper layers and a plurality of graphene layers; and afeeding unit to feed the spiral through the opening of the die unit.

In some examples, the die unit and/or the extracting unit and/or thefeeding unit may be separate units.

In other examples, some or all of the die unit and/or extracting unitand/or feeding unit may be combined into a common unit.

In particular, the extracting unit may comprise the feeding unit. Acombined extracting-feeding unit may comprise a pushing unit to push thewrapped sheet towards and against the die unit and through the openingof the die unit, thereby extracting an inner layer of small diameter andforming a spiral.

The opening may have a diameter smaller than a diameter of the wrappedsheet or spiral, but larger than a diameter of the wrapped inner layer.In particular, this may allow extracting the inner layer of the wrappedsheet by pushing the wrapped sheet against the boundary of the opening,and thereby extracting the inner layer through the opening.

A pulling unit pulling the inner layer from the opposite side of theopening may assist in the extracting of the inner layer and the feedingof the spiral through the opening.

The opening may have an elliptic cross section, in particular a circularcross section.

The opening may have an inclination angle with respect to a feedingdirection of the spiral.

In an example, the die unit comprises a first opening and a secondopening downstream of the first opening in a feeding direction, whereinthe second opening is smaller than the first opening.

The die unit may additionally comprise a third opening downstream of thesecond opening in the feeding direction, wherein the third opening issmaller than the second opening.

The first opening may have a first inclination angle with respect to afeeding direction of the spiral.

The second opening may have a second inclination angle with respect to afeeding direction of the spiral.

The third opening may have a third inclination angle with respect to afeeding direction of the spiral.

The first, second, and third inclination angles may be identical, or maydiffer from one another.

In an example, the first inclination angle and/or the second inclinationangle and/or the third inclination angle is no smaller than 5°, inparticular no smaller than 10° or 15°.

In an example, the first inclination angle and/or the second inclinationangle and/or the third inclination angle is no larger than 30°, inparticular no larger than 25° or 20°.

In an example, the feeding unit comprises a pushing unit to push thespiral through the opening.

The feeding unit may alternatively or additionally comprise a pullingunit to pull the spiral through the opening.

In an example, the pulling unit comprises a plurality of pulling rolls,in particular a plurality of rotating pulling rolls.

In an example, the pulling rolls are configured to orbit around thespiral, in particular around a longitudinal axis of the spiral.

The features of the wire-drawing method and wire-drawing system havebeen described above in isolation. However, in some examples some or allof the features as described above may be combined.

Wire-drawing method and systems according to the disclosure will now bedescribed with reference to composite structures comprising copperlayers sandwiched by graphene layers, and multilayer structures formedby stacking such composite structures.

However, this is merely an example, and the disclosure relates todrawing a wire from any wrapped sheet comprising a plurality of copperlayers and a plurality of graphene layers.

BRIEF DESCRIPTION OF THE FIGURES

The effects and numerous advantages of the present disclosure willbecome clearer from a detailed description of examples with reference tothe accompanying drawings, in which:

FIG. 1 is a schematic cross-section of a composite structure accordingto an example;

FIG. 2 is a schematic cross-section illustrating electron tunneling in acomposite structure according to an example;

FIG. 3 is a cross-sectional schematic illustration of a compositebi-layer structure according to an example;

FIG. 4 is a flow diagram illustrating a method for forming a compositestructure according to an example;

FIG. 5 is a schematic illustration of a system for forming a compositestructure according to an example;

FIG. 6 is a schematic illustration of a transport unit withcounter-rotating rollers according to an example;

FIG. 7 is a schematic illustration of a system for forming a compositestructure employing plasma tubes according to an example;

FIG. 8 is a schematic illustration of a system for forming a compositestructure according to an example;

FIG. 9 is a schematic cross-sectional illustration to illustrate theeffect of grain boundary scattering in a composite structure accordingto an example;

FIG. 10 is a schematic drawing illustrating the effect of coating andgrain coarsening according to an example;

FIGS. 11a and 11b illustrate a hexa-hepta functionalization of graphenelayers with metal atoms according to an example;

FIG. 12 is a flow diagram illustrating a method for forming a multilayercomposite structure according to an example;

FIG. 13 is a cycle diagram illustrating a method for forming amultilayer composite structure according to an example;

FIG. 14 is a schematic illustration of a system for forming a multilayercomposite structure according to an example;

FIG. 15 is a schematic illustration of a system for forming a multilayercomposite structure according to another example;

FIG. 16 illustrates a wrapped coated sheet according to an example;

FIGS. 17a and 17b schematically illustrate a compacting unit accordingto an example;

FIGS. 18a to 18c schematically illustrate the effect of compacting awrapped coated sheet according to an example;

FIG. 19 is a flow diagram illustrating a wire-drawing method accordingto an example;

FIG. 20 is a schematic illustration of a wire-drawing system accordingto an example;

FIG. 21 is a perspective schematic illustration of a composite wrapextracted to form a band spiral according to an example;

FIGS. 22a and 22b illustrate the functioning of a wire-drawing systemaccording to an example;

FIGS. 23a to 23c illustrate different stacking modes in bi-layergraphene structures according to an example; and

FIGS. 24a and 24b illustrate the transformation between differentstacking modes of bi-layer graphene structures according to an example.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration of the composite structure 10according to an example in a cross-sectional view. The compositestructure 10 comprises a copper layer 12 sandwiched by a first (lower)graphene layer 14 a and a second (upper) graphene layer 14 b.

The copper layer 12 may be formed of pure copper (Cu), in particular ina Cu (111) crystallographic orientation. However, in other examples, thecopper layer comprises additional material other than copper, such asdoped atoms or nanoparticles, as will be described in more detail below.

The copper layer 12 as shown in FIG. 1 extends at a width or thicknessd₁ between the first graphene layer 14 a and the second graphene layer14 b, wherein d₁ is generally no larger than 25 μm. For instance, thecopper layer 12 may be formed at a thickness of 20 μm or 10 μm. In otherexamples, the copper layer 12 may be formed at a thickness of no largerthan 2 μm or no larger than 1 μm.

The first graphene layer 14 a and second graphene layer 14 b aregenerally much thinner than the copper layer 12, and may in particularbe graphene layers comprising a single graphene monolayer, or aplurality of graphene monolayers. In FIG. 1, the width or thickness ofthe graphene layers 14 a and 14 b is denoted by d_(a) and d_(b),respectively. For instance, d_(a) and d_(b) may both amount to 5 nm, 2nm, or even less.

In a graphene monolayer configuration, the conduction band touches thevalence band in single points, the so-called Dirac points. Theinfinitesimal small band gap explains the superior electric conductivityof graphene monolayer structures as opposed to graphite.

The first graphene layer 14 a and the second graphene layer 14 b may bechemically connected or bound to the copper layer 12, such as depositedon the copper layer 12 by means of chemical vapor deposition, as will bedescribed in additional detail below.

The schematic illustration of FIG. 1, the width directions d₁, d_(a),and d_(b) define the z direction. The copper layer 12 and first andsecond graphene layers 14 a, 14 b may spatially extend widely in theplanar spatial directions x, y perpendicular to the width direction z,so that the composite structure 10 forms a thin foil, as will bedescribed in additional detail below.

Graphene coatings on copper are known to have remarkable in-planestiffness to prevent mechanical deformation, such as surfacedistortions. Moreover, graphene coatings show impermeability to protectcopper against reactive chemical or gaseous species, along with lowdensity-of-states to avoid perturbation of the copper surface potentialafter coating. All of these superior physical properties make graphenean ideal non-interacting barrier for copper samples, prevent oxidationand surface contamination in general, and contribute to a compositestructure with superior parameters.

FIG. 1 further illustrates schematically the path 16 of an electron etraveling through the copper layer 12. As can be taken from FIG. 1, theelectron e is repeatedly scattered off the boundaries between the copperlayer 12 and first graphene layer 14 a and second graphene layer 14 b,respectively. The graphene layers 14 a, 14 b protect the surface of thecopper layer 12 to result in a largely smooth, pristine and wrinkle-freelayer. In particular, graphene grown on Cu (111) surfaces is only weaklyinteracting with the Cu surface states. The relatively weak couplingbetween the carbon 2 p orbitals of graphene and the Cu 4 p orbitalsleads to predominantly elastic surface scattering, which reduces theelectric conductivity of the composite structure 10 in the planerdirection x, y.

The graphene layers 14 a, 14 b in addition improve the thermalconductivity of the composite structure 10. In general, thermal devicedesign is becoming an important part of electric and microprocessorengineering. The high thermal conductivity of the graphene layers 14 a,14 b can provide a parallel phonon component for heat conduction.

In addition, the graphene layers 14 a, 14 b enhance the mechanicalstrength of the composite structure 10 in the planer direction x, y.

FIG. 2 is a schematic cross-sectional drawing of the composite structure10 that generally corresponds to FIG. 1, but illustrates a tunnelingmechanism in which electrons tunnel through part of the graphene layers14 a, 14 b to result in even higher electric conductivity. A compositestructure 10 with a thin copper layer 12, in particular a copper layer12 whose width d₁ is in the range of only a few electron mean free pathlengths in copper, presents a waveguide-like structure where electronsexperience only a fraction of the bulk electrical resistance. Thiseffect exploits the wave nature of the electrons in a quantumelectrodynamic description. In particular, electrons traveling throughthe copper layer 12 may tunnel through electron tunnels 18 that form inthe graphene layers 14 a, 14 b. Since the electrons travel through anideal graphene monolayer 14 a, 14 b close to the speed of light whiletheir speed in the copper layer 12 is much lower in general, and alsolimited due to various scattering effects, the electron wave approachingthe copper/graphene interface appears to be back-scattered not at thelocation of the impact but further down in the flow direction (to theright in FIG. 2). As shown in the electron path 16 in FIG. 2, theelectron is still back-scattered from the graphene layer 14 b into thecopper layer 12, but with a displacement corresponding to the distanceit traveled (tunneled) through the graphene layer 14 b. Because of thelarge difference in electron speed between copper and graphene in aquantum approach, an electron e travels a certain distance within thecomposite structure 10 much faster than in a classical particleapproach. Given that a higher electron speed is generally equivalent toa better electrical conductivity, the possibility of electron tunnelingin a composite structure 10 with a thin copper layer 12 sandwiched bygraphene layers 14 a, 14 b leads to an enhanced electric conductivity.

The effect may be explained in additional detail in terms of the Kleineffect: A peculiar property of the Dirac Hamiltonian is that chargecarriers (like electrons) cannot be confined by electrostaticpotentials. In conventional conductors, if an electron strikes anelectrostatic barrier that has a potential height above the electron'skinetic energy, the electron wave function becomes evanescent within thebarrier, and exponentially decays with increasing distance into thebarrier. This means that the taller and wider the barrier is, the morethe electron wave function will decay before reaching the other side.Thus, usually, the taller and wider the barrier is, the lower theprobability of the electron quantum tunneling through the barrier.

However, if the particles are governed by the Dirac equation, theirtransmission probability actually increases with increasing barrierheight. A Dirac electron that hits a tall barrier may turn into a holeand propagate through the barrier until it reaches the other side, whereit may turn back into an electron. This phenomenon is sometimes calledKlein tunneling.

One possible explanation for this phenomenon is that increasing barrierheight may lead to an increased degree of mode matching between the wavefunctions of the holes within the barrier and the electrons outside ofit. When the modes are perfectly matched (which may correspond to thecase of an infinitely tall barrier), there may be perfect transmissionthrough the barrier.

Applying these considerations to the composite structure 10 as shown inFIG. 2, the graphene layers 14 a, 14 b may be thin enough to appearelectrically invisible, allowing an electron to “see” through thegraphene the copper layer on the other side of the tunnel. As soon asthe electron is inside the electron tunnel 18, it experiences the muchhigher conductivity and speeds up in the direction of the main currentflow.

As illustrated in the conceptual cross-sectional diagram of FIG. 3,several composite structures 10 as described above with reference toFIGS. 1 and 2 may be stacked to form a multilayer composite structure20. In the stacked multilayer composite structure 20, a first graphenelayer 14 a of a first composite structure 10 is positioned on and indirect contact with the second graphene layer 14 b′ of another compositestructure 10′.

FIG. 3 shows a multilayer composite structure 20 with two elementarycomposite structures 10, 10′, but in general a large number of compositestructures may be stacked in the same way.

As can be taken from FIG. 3, the multilayer composite structure 20comprises a graphene bi-layer 22 formed of a pair of graphene layers 14a, 14 b′ of neighboring composite structures 10, 10′.

The graphene bi-layer 22 takes particular advantage of the Klein effectas described above, since it allows electrons to tunnel back and forthbetween copper layers 12 of neighboring composite structures 10 throughthe electron tunnels 18, 18′ formed in the graphene layers 14 a, 14 b′.Figuratively speaking, the adjacent graphene layers 14 a, 14 b′ of thegraphene bi-layer 22 form a high-speed tunnel path through themultilayer composite structure 20, which results in a superior electricconductivity.

The effect may be particularly pronounced in multilayer compositestructures 20 with only exactly two adjacent graphene layers 14 a, 14b′. With three or more layers, at least one graphene layer would not bedirectly connected to copper.

The composite structure 10 and multilayer composite structure 20 asdescribed with reference to FIGS. 1 to 3 above may be employed to formelectrical cables or wires with enhanced electrical conductivity.

FIG. 4 is a schematic flow diagram illustrating a method for forming acomposite structure.

In a first step S10, a copper foil having a first surface side and asecond surface side opposite the first surface side is provided, whereinthe copper foil has a thickness of no larger than 25 μm.

In a second step S12, a first graphene layer is deposited on the firstsurface side of the copper foil, and a second graphene layer isdeposited on the second surface side of the copper foil.

A plurality of coating techniques may be employed to deposit the firstgraphene layer and the second graphene layer on the copper foil. Forinstance, the depositing may comprise chemical vapor depositiontechniques. A chemical vapor deposition (CVD) apparatus 24 isschematically illustrated in FIG. 5. A corresponding apparatus isdescribed in greater detail by Polson, McNerney, Viswanath, et al. inNature Scientific Reports, May 2015, 10.1038/srep 10257.

The CVD apparatus 24 as shown in FIG. 5 comprises a transport unit witha plurality of transport rolls 26 a, 26 b to transport a thin copperfoil 28 subsequently through an annealing zone 30 and a growth zone 32downstream of the annealing zone 30.

In the annealing zone 30, the copper foil 28 may be heated totemperatures in the range between 500° C. and 1000° C. in the presenceof a pure argon gas flow (indicated by arrows in FIG. 5). By keeping thecopper foil 28 for 20 to 30 minutes at elevated temperatures, the copperfoil 28 is annealed, resulting in a considerable surface smoothening.Moreover, grain growth can be initiated.

Chemical vapor deposition takes place in the growth zone 32, wherecarbon atoms supplied by a precursor gas such as methane are depositedas the surface of the copper foil 28. The flow of the precursor gas islikewise illustrated by arrows in FIG. 5. Heating wires 34 a, 34 b arearranged along the length of the annealing zone 30 and growth zone 34 toprovide the annealing temperature and break up the molecules of theprecursor gas.

Graphene synthesis under atmospheric CVD conditions may be performed inthe growth zone 32 at temperatures between 500° C. and 1000° C. using agas mixture of argon and methane. Monolayer graphene growth up to 96% ofthe total area could be observed when a methane gas concentrationvarying between 0.2% and 1% by volume was employed. Higher methaneconcentrations may lead to the formation of multi-layer graphenestructures.

It was also observed that the formation of the graphene monolayer on thecopper foil 28 at elevated temperatures led to the preferred formationof a Cu (111) crystallographic orientation structure on the coppersurface. In some instances, the graphene layer may act as a sort oftemplate to the copper surface of the copper foil 28, which promotes theformation of a Cu (111) crystallographic orientation or copper structurebecause of the close lattice size match between copper and graphene inthis configuration.

The CVD apparatus 24 as illustrated in FIG. 5 may be employed to deposita graphene layer on a single surface side of the copper foil 28. Inorder to deposit graphene layers 14 a, 14 b on both sides of the copperfoil 28 to arrive at a composite structure 10 as described withreference to FIGS. 1 and 2, one may pass the copper foil 28 through theCVD apparatus 24 twice, with opposite surface orientations of the copperfoil 28.

The surface orientation may be changed by means of a transport unitcomprising a pair of counter-rotating transport rolls 36 a, 36 b asshown in FIG. 6. For instance, a first transport roll 36 a may rotate ina first (clockwise) direction and may pass the copper foil 28 to asecond transport roll 36 b rotating in an opposite (counter-clockwise)direction to change the orientation of the copper foil 28.

Optionally, the copper foil 28 may be etched before being fed into theCVD apparatus 24 to remove surface contaminants, such as by exposing thesamples at 450° C. for 90 minutes to a hydrogen (70 vol.-%) and argon(30 vol.-%) gas flow.

FIG. 7 is a schematic illustration of a chemical vapor deposition (CVD)apparatus according to another example that may be employed for formingthe composite structures of the present disclosure. The CVD apparatus 38illustrated in FIG. 7 is generally similar to the CVD apparatus 24described above with reference to FIG. 5, but employs coldplasma-enhanced chemical vapor deposition rather than electrical heatingby means of heating wires 34 a, 34 b.

In greater detail, the cold plasma CVD apparatus 38 comprises atransport unit with transport rolls 26 a, 26 b to transport a thincopper foil 28 through a growth zone 32. A supply unit (not shown in theschematic illustration of FIG. 7) directs a precursor gas comprisingcarbon into the growth zone 32 so that the precursor gas passes andcirculates around the copper foil 28 on both of its opposing surfacesides, as indicated by the arrows in FIG. 7. For instance, the precursorgas may comprise methane and/or butane and/or propane.

As can be further taken from FIG. 7, a plurality of plasma units 40comprising plasma tubes are provided in the growth zone 32 on both sidesof the copper foil 28 to generate a cold plasma discharge that breaks upthe molecules of the precursor gas and allows the carbon atoms to coatboth surface sides of the copper foil to form the first and secondgraphene layers 14 a, 14 b. As can be taken from FIG. 7 and the abovedescription, the cold plasma CVD apparatus 38 allows for the graphenelayers 14 a, 14 b to be deposited on both surface sides of the copperfoil 28 simultaneously, thereby enhancing the process efficiency andreducing cycle times.

Preceding etching and/or annealing may take place in the same way asdescribed above with reference to FIG. 5.

A cold plasma CVD apparatus 38′ according to another example isschematically illustrated in the perspective view of FIG. 8.

As can be taken from FIG. 8, the cold plasma CVD apparatus 38′ comprisesa transport unit with a plurality of transport rolls 42 a to 42 e thattransport a copper foil 28 through a growth zone 32, similarly to theconfiguration of FIG. 7. A supply unit 44 supplies a precursor gas suchas methane and directs it through the growth zone 32 so that it passesand circulates around the copper foil 28 on both surface sides, asindicated by the arrows in FIG. 8. A deposition unit comprising aplurality of plasma units 40 with plasma tubes provides a cold plasmadischarge to break up the molecules of the precursor gas so that thecarbon is deposited on both surface sides of the copper foil as firstand second graphene layers 14 a, 14 b.

However, unlike the configuration of FIG. 7 in which the copper foil 28is transported horizontally through the growth zone 32, in theconfiguration of FIG. 8 the transport rolls 42 a to 42 e transport thecopper foil 28 through the growth zone 32 in a vertical direction, i.e.in a direction that corresponds to the direction of the gravitationalfield in which the CVD apparatus 38′ is placed. This allows to coat bothsurface sides of the copper foil 28 more uniformly.

Preceding etching and/or annealing may take place in the same way asdescribed above with reference to FIG. 5.

As can be further taken from FIG. 8, the cold plasma CVD apparatus 38′additionally comprises a charging unit 46 electrically connected to thetransport rolls 42 b, 42C to electrically charge the copper foil 28.Moreover, the charging unit 46 is electrically coupled to the plasmaunits 40 to charge the precursor gas with a charge opposite from thecharge of the copper foil 28. The electrostatic charging of the copperfoil 28 facilitates the growth of the graphene layers 14 a, 14 b on thecopper foil 28, and may lead to graphene layers 14 a, 14 b that are morehomogeneous and uniform.

In contrast to more conventional graphene growth in a furnace thatreaches temperatures between 900 and 1000® C., the cold plasma coatingtechnology enables graphene deposition at a much lower temperature,which may be in the range of approximately 650° C. The reducedtemperature leads to reduced thermal stress of the copper surface, andprevents damage and crystallographic reconfiguration of the surface ofthe copper foil that could be detrimental to an efficient graphenecoating.

At the same time, the temperatures reached in the cold plasma coatingtechnology are sufficiently high to provide for a controlled andthorough grain coarsening of the copper layer 12 of the compositestructure 10. It may be advantageous to start with a copper layer 12that already has a majority Cu (111) crystallographic orientation orgrain orientation, which may subsequently transform into complete Cu(111) crystallographic orientation or coarse grain orientation aftergraphene deposition, supported by the good lattice match between the Cuatomic spacing and the graphene lattice constant.

FIG. 9 is a schematic illustration of a composite structure 10 thatgenerally correspond to the composite structure illustrated in FIGS. 1and 2 above, but in addition depicts the grain boundaries 48 that mayexist in the copper layer 12 and may lead to inelastic electronscattering that can hamper the electric conductivity of the compositestructure 10. The thermal energy supplied in the course of the coldplasma coating process may lead to a grain coarsening, i.e. to a largergrain size of at least 10 μm or greater, as illustrated schematically inFIG. 10. The larger grain size may entail a reduction of inelasticscattering of electrons inside the copper layer 12, and may hencecontribute to a higher electric conductivity.

The relatively low bonding strength between individual layers of thecomposite structure 10 or multilayer composite structure 20 may beenhanced by chemically functionalizing graphene, such as by hexa-heptafunctionalization. In particular, plasmonic metal particles, such assilver nanoparticles, may be added to the graphene layers 14 a, 14 bwithout changing the hexagonal lattice properties. Instead of addingmolecules to the individual carbon atoms of graphene (covalentfunctionalization), hexa-hepta functionalization or ring-centeredfunctionalization adds metal atoms, such as silver nanoparticles to thecenter of the graphene ring. The bond is delocalized inside the graphenering, which keeps the hexagonal arrangement undistorted, so thatgraphene retains its unique electrical properties. At the same time, themetal atoms may serve as anchoring points for thermally connecting thegraphene layers 14 a, 14 b to the underlying copper layer 12.

FIGS. 11a and 11b are a plane view and a perspective view, respectivelyof a graphene monolayer 50 with the characteristic hexagonal carbonrings and show the doped ring-centered silver nanoparticles 52delocalized inside the carbon rings.

The hexa-hepta functionalization can be performed after CVD growing agraphene monolayer on a pristine copper Cu (111) crystallographicorientation surface as described above. It may consist of a two-stepprocess; a synthesis of η⁶-graphene Cr(Co)₃ followed by silvernanoparticles attachment. Corresponding techniques have been describedby S. Che et al. in “Retained Carrier-Mobility and EnhancedPlasmonic-Photovoltaics of Graphene via Ring-Centered Functionalizationand Nano-Interfacing”, Nano Letters, Jun. 6, 2017.

Forming Multilayer Composite Structures

The manufacturing techniques described above provide a compositestructure 10 with a single copper layer 12 sandwiched by first andsecond graphene layers 14 a, 14 b, as illustrated in FIGS. 1 and 2. Inorder to provide the multilayer composite structures 20 as illustratedin FIG. 3 with their enhanced electrical conductivity, the coated copperfoil 28 may be stacked.

In an example, the coated copper foil 28 may be cut into pieces orstripes, and several of these pieces may be stacked under pressure andheat.

In an example, pressing may involve sintering, such as hot sintering,microwave sintering or field-assisted sintering involving alternatingcurrent or direct current.

For instance, sintering temperatures may be in the range between 500° C.and 1000° C. By a suitable choice of the sintering temperature andsintering duration, the copper grain size may be further enhanced.Moreover, sintering allows the copper layers to better adapt to thegraphene crystal structure, and hence fosters the formation of acrystallographic Cu (111) crystallographic orientation surface structurein the copper of the copper foil 28.

The stacking may be performed in a combined sintering and pressingapparatus under mechanical pressures in the range of 10 MPa to 300 MPa.

In an alternative configuration that will now be described in additionaldetail with reference to FIGS. 12 to 18, a multilayer compositestructure 20 may be provided by wrapping and compacting the coatedcopper foil 28.

FIG. 12 is a flow diagram illustrating a method for forming a multilayercomposite structure 20 comprising a stack of composite structures 10according to an example.

In a first step S20, a first sheet comprising a copper-comprising layersandwiched by first and second graphene layers is provided, such as thecoated copper foil 28.

In a second step S22, the first sheet is wrapped to form a first rod.

In a third step S24, the first rod is compacted to form a firstmultilayer composite structure.

The sequence of steps as illustrated in FIG. 12 may be repeated severaltimes. In particular, the multilayer composite structure resulting fromstep S24 may again be coated on both sides by back feeding the resultingmultilayer foil into the cold plasma CVD apparatus 38, 38′ as describedabove with reference to FIGS. 7 and 8, respectively, followed by asecond round or several additional rounds of wrapping and compacting.

An exemplary process cycle is illustrated in FIG. 13.

In step 1, graphene layers are deposited by means of chemical vapordeposition on both sides of a copper-comprising foil, such as byemploying a cold plasma CVD apparatus 38, 38′.

In a subsequent step 2, the resulting foil is rolled or wrapped to formrolled graphene bi-layers.

In a subsequent step 3, the resulting rod is roll-milled to convert thebi-layer roll to a bi-layer foil.

In a subsequent (optional) step 4, the outer surfaces of the bi-layerfoil may be etched to provide a pristine copper surface, before the foilis back fed into the graphene CVD facility in step 5.

The process cycle can be described by the following sequence of steps:

-   -   0. A counter is set to zero.    -   1. A graphene monolayer is grown on both sides of a        copper-comprising foil in a CVD coating facility.    -   2. The coated foil is then wrapped around so that a tube or rod        is created with a circular concentric structure of alternating        copper and bi-layer graphene layers.    -   3. The rod is then compacted, such as by metal hot rolling        pressing, creating a flat copper/graphene multilayer composite        band containing a multitude or plurality of graphene bi-layers        22.    -   4. The resulting composite band is then etched on both surfaces        to provide a pristine copper surface.    -   5. The counter is now increased by 1. If the counter is smaller        than a pre-defined threshold, the process proceeds with step 1        by back-feeding the resulting structure into the CVD coating        facility.    -   6. If the counter reaches the threshold value, the        copper/graphene composite band is extracted from the coating and        rolling cycle.

The process circle as described above may be called “CWH-Circle”(coating, wrapping, hot-rolling). The process circle may result in amultilayer composite structure in which the volume fraction of grapheneis significantly enhanced compared to the single-layer compositestructure 10 of FIG. 1. In particular, the process circle allows to varythe volume fraction of graphene in the matrix by selecting the number ofcircle iterations. This may allow to increase and/or tailor theelectrical conductivity, the thermal conductivity, and/or the mechanicalstrength of the copper/graphene composite material to a desired value.

FIG. 14 is a schematic illustration of a system 54 for forming amultilayer composite structure employing the techniques described above.

The system 54 comprises a transport unit 56, such as a plurality ofdriven transport rolls, to transport a sheet 58, such as the copper foil28, to a deposition unit 60.

The deposition unit 60 may comprise a cold plasma CVD apparatus 38, 38′as described with reference to FIGS. 7 and 8 above, and may depositpairs of graphene layers 14 a, 14 b on opposing first and second surfacesides of the sheet 58, resulting in a coated sheet 62 with a compositestructure 10 as described above with reference to FIGS. 1 and 2.

In an example, the deposition unit 60 may comprise an annealing unit,providing an annealing zone 30 upstream of the deposition growth zone asdescribed above with reference to FIG. 5.

The transport unit 56 transports the coated sheet 62 from the depositionunit 60 to a wrapping unit 64 to wrap the coated sheet 62 into a rod 66.The wrapping unit 64 may employ any technique to wrap the coated sheet,such as rolling up the coated sheet on a thin cylindrical roll, andremoving the roll.

The resulting rod 66 is illustrated schematically in FIG. 16. As can betaken from FIG. 16, due to the wrapping or rolling, the rod 66 comprisesgraphene bi-layer structures in which two graphene layers 14 a, 14 b arein immediate and direct contact, as described above with reference toFIG. 3, and therefore has all the conductivity advantages describedabove.

With further reference to FIG. 14, the transport unit 56 may transportthe rod 66 to a compacting unit 68 downstream of the wrapping unit 64.The compacting unit 68 may compact the rod 66 into a compacted sheet 70.

For instance, the compacting unit 68 may be a rolling press, inparticular a hot rolling press, as illustrated schematically in a frontview in FIG. 17a and in a side view in FIG. 17 b.

As can be taken from FIGS. 17a and 17b , the hot rolling press 68 maycomprise a plurality of press units 72 a, 72 b, 72 c in a staggeredconfiguration, so that the rod 66 passes subsequently through the pressunits 72 a, 72 in a transport direction (illustrated by the centralarrow in FIGS. 17a and 17b ) and is thereby converted into the compactedsheet 70, such as a thin long band.

FIGS. 17a and 17b show a compacting unit 68 with three staggered pressunits 72 a, 72 b, 72 c. However, this is for illustration only, and inother configurations the compacting unit 68 may comprise a smaller orlarger number of press units.

The compacting unit 68 may further comprise a heating unit (not shown)to heat the rod 66 during the compacting. Hot rolling may provide twoadvantageous effects: On the one hand, it further increases the coppergrain size, which leads to a reduction of the grain scattering and henceto superior electric conductivity, as described above with reference toFIGS. 9 and 10. On the other hand, hot rolling may foster the copper Cu(111) crystallographic orientation, which may increase the lattice matchbetween the copper surface and the grown-on graphene layers, therebyproviding a multilayer composite structure with enhanced mechanicalproperties.

For grain growth to happen effectively, the processing temperatureshould be sufficiently higher than the copper re-crystallizationtemperature of approximately 227° C. However, in order to avoid, reduceor minimize nano-cracks of the copper surface, in some examples the hotrolling temperature should be chosen below 650° C. In particular, thehot rolling temperature may be chosen between 450° C. and 550° C.

The sequence of FIGS. 18a to 18c show the rod 66 in a cross-sectionalschematic view and its transformation into the compacted sheet 70 as itprogresses through the press units 72 a, 72 b, 72 c of the compactingunit 68.

The techniques described above take advantage of the fact that graphenelayers are sort of “slippery”, and generally do not stick to each otherwell. The reason behind this effect is that graphene has strong covalentbonding between its atoms in the horizontal direction (in-plane with themonolayers) but only relatively weak van der Waals forces in a directionvertical to the in-plane direction, which keeps it from mechanicallyattaching vertically to the next layer. On the other hand, because ofthe excellent match between the graphene lattice constant and the atomicspacing of the Cu (111) crystalline orientation, the graphene layer isstrongly mechanically bonded to the copper surface on which it is grown.

Due to these reasons, graphene/copper layers can slide relatively easilywith respect to each other, making the above-described conversion from around shape of the rod 66 to a flat band 70 possible without sacrificingthe excellent mechanical connection between the individual graphenelayers 14 a, 14 b and the copper layers 12 on which they are grown. Atthe same time, the copper surfaces are protected by the mechanicallyextremely robust graphene coating, keeping any mechanical stress awayfrom the copper during the severe plastic deformation. As illustrated inthe insert of FIG. 18c , the result is a multilayer composite structure20 corresponding to a stacking of the composite structure 10 asdescribed above with reference to FIG. 3.

As can be further taken from FIGS. 18b and 18c , the plurality ofgraphene layers in the compacted sheet 70 are still interconnected withone another, which is due to the fact that the compacted sheet 70results from compactification of the rod 66 having a single wrappedgraphene layer. The connection fosters the electric conductivity in adirection perpendicular to the in-plane direction.

With further reference to FIG. 14, the system 54 additionally comprisesa back-feeding unit 74 which may comprise transfer rollers to back feedthe compacted sheet 70 to the deposition unit 60 for another round ofchemical vapor deposition, wrapping, and compacting, as described abovewith reference to the circle diagram of FIG. 13.

FIG. 15 is a schematic illustration of a system 54′ for forming amultilayer composite structure according to another example. The system54′ is similar in design and functionality to the system 54 describedwith reference to FIG. 14 above, and the same reference numerals areemployed to denote corresponding components. However, the system 54′comprises one or a plurality of optional units, which are illustrated inbroken lines in FIG. 15.

In particular, the system 54 may comprise an etching unit 76 upstream ofthe deposition unit 60. The etching unit 76 may be employed to etch afirst surface side and/or a second surface side of the sheet 58 and/orthe back-fed compacted sheet 70 by means of chemical etching, so as toprovide a pristine copper surface for a subsequent deposition in thedeposition unit 60.

For instance, the etching unit 76 may comprise a quartz tube furnace,which may expose the sheet 58 to a hydrogen (70 vol.-%) and argon (30vol.-%) gas flow at temperatures in the range of 450° C. forapproximately 90 minutes. This allows to efficiently remove surfacecontaminants from the surfaces of the sheet 58.

As can be further taken from FIG. 15, the system 54 may alternatively oradditionally comprise a bypass unit 78 configured to bypass the sheet 58or compacted sheet 70 past the deposition unit 60.

In particular, after the circle process has reached the desired numberof iterations, the resulting multilayer compacted sheet 70 may be fedback to the circle by means of the back-feeding unit 74 one last time,but skipping the graphene coating step. In particular, the bypass unit78 may pass the compacted sheet to the wrapping unit 64 so that it doesnot traverse the deposition unit 60. This may ensure that in the finalwrapped rod 66 produced by the wrapping unit 64, the single layers arenot separated by graphene layers. In particular, this may facilitatelayer fusing in a subsequent wire drawing technique in a wire-drawingsystem 80, as will be described in additional detail with reference toFIGS. 19 to 24 below. Wire drawing in a wire-drawing system 80 may be afinal step to convert the rod 66 into a wire 82 with enhancedelectrical, thermal and mechanical properties.

Alternatively, as further illustrated in FIG. 15, the system 54 maycomprise a slicing unit 84 downstream of the compacting unit 68 andadapted to slice the compacted sheet 70 in a longitudinal direction intoa plurality of elongated slices 86 in a final process step, after themaximum number of iterations has been reached. This may provide wireswith a rectangular or square cross-section.

Wire-Drawing Method and System

As described above with reference to FIG. 15, wire drawing techniquesmay be employed to convert a multilayer composite rod 66 comprising themultilayer composite structure 20 into a wire.

FIG. 19 is a flow diagram illustrating a wire-drawing method accordingto an example.

In a first step S30, a rod comprising a wrapped sheet is provided,wherein the sheet comprises a plurality of copper layers and a pluralityof graphene layers.

In a second step S32, an inner layer of the wrapped sheet is extractedfrom the rod to form a spiral or helix.

In a third step S34, a wire is formed by feeding the spiral through anopening of a die unit.

FIG. 20 is a schematic illustration of a corresponding wire-drawingsystem 80 that may be configured to perform the steps of the methodillustrated in the flow diagram of FIG. 19.

The wire-drawing system 80 comprises an extracting unit 88 adapted toextract an inner layer of a wrapped sheet to form a spiral, wherein thesheet comprises a plurality of copper layers and a plurality of graphenelayers.

In particular, the wrapped sheet may be the rod 66 comprising themultilayer composite structure as described above with reference to FIG.18a . The extracting unit 88 may be adapted to extract an innermostlayer of the rod 66, such as by grabbing the innermost layer of the rod66 and pulling it to form a helix or spiral 90 as illustrated in theperspective drawing of FIG. 21. In the spiral 90, each individualgraphene bi-layer 22 is connected on both sides directly to the coppersubstrate where they are grown, so that the enhanced conductivity occursin a largely increased volume fraction of the wire, providing for bothultra-high conductivity and high current capacity.

As described above with reference to FIG. 15, it may be beneficial thatthe windings of the spiral 90 do not have graphene-coating on theoutside, since this might prevent them from fusing togetherhomogeneously. In addition, a graphene-free surface provides optimumconditions for connecting the resulting wire 82 to sockets or plugs,such as by means of soldering, clamping, crimping, etc.

With additional reference to FIG. 20, the wire-drawing system 80 furthercomprises a feeding unit 92 downstream of the extracting unit 88, andadapted to feed the spiral 90 to a die unit 94, which converts thespiral 90 into a wire 82.

In some examples, the feeding unit 92 may comprise a pushing unitconfigured to push the spiral 90 towards and through the die unit 94.

In some examples, the extracting unit and the feeding unit are separateunits.

In other examples, the feeding unit may be part of the extracting unit.For instance, in a configuration in which the feeding units comprises apushing unit to push the spiral towards and through the die unit 94, thepushing may extract an inner layer of the wrapped sheet as a result ofthe pushing force.

FIG. 22a is a schematic side view of a die unit 94 as it may be used inthe wire-drawing system 80. The die unit 94 comprises a plurality ofdies 96 a, 96 b, 96 c in a staggered configuration. Each of the dies 96a, 96 b, 96 c comprises a respective opening 98 a, 98 b, 98 c, wherein adiameter or surface area of the openings 98 a, 98 b, 98 c decreases in atransport direction (to the right in FIG. 22a ) of the spiral 90 as itpasses through the die unit 94. The openings 98 a, 98 b, 98 c may beopenings with a circular or elliptic cross-section, depending on thedesired cross-section of the wire 82. The openings 98 a, 98 b, 98 c mayhave a diameter smaller than a diameter of the rod 66 or spiral 90, butlarger than the wrapped innermost layer and thereby subsequently deformthe rod 66 and spiral 90 into a wire under the pulling and/or pushingforce.

FIG. 22a further illustrates a pulling unit 100 comprising a pluralityof pulling rolls 102 a, 102 b that pull the spiral 90 through theopenings 98 a, 98 b, 98 c of the die unit 94. The pulling unit 100 mayform part of the feeding unit 92, or may be a separate component.

As can be further taken from FIG. 22b , the pulling rolls 102 a, 102 bmay be configured to rotate or orbit around the drawn wire with apre-selectable spacing in between to create a wire 82 with a desiredouter diameter and shape.

The configuration of FIGS. 22a and 22b shows a die unit 94 with threedies 96 a, 96 b, 96 c with openings of decreasing diameter in thetransport direction of the spiral 90. However, in general, the die unit94 may comprise any number of dies in a staggered configuration.

The wire-drawing techniques according to the disclosure may also beemployed for additional conductivity tuning of the composite multilayerstructures, as will now be described with reference to FIGS. 23 and 24.

Stacked graphene, in which at least two graphene monolayers 104 a, 104 bare stacked with a twist angle Θ between them, may exhibit uniqueelectronic, thermal, and magnetic properties. A rotational twist of thegraphene monolayers 104 a, 104 b with respect to one another can have aprofound effect on the electrical properties of the bi-layer structure.Controlling the twist angle Θ of bi-layer graphene films hence allowsfor the preparation of twisted bi-layer graphene films with definedstacking orientations, and in turn the tailoring and fine-tuning oftheir electronic, thermal, and magnetic properties.

FIGS. 23a to 23c illustrate three different basic stacking modes inbi-layer graphene:

AB-stacking (also called Bernal stacking) is illustrated in FIG. 23a .The second graphene layer 104 b is displaced by one half of the diameterof the hexagonal ring structure with respect to the hexagons of thefirst graphene layer 104 a. This results in a parallel shift without atwist.

FIG. 23b schematically illustrates the so-called AA-stacking (alsocalled non-Bernal stacking), in which the hexagons of the two layers 104a, 104 b lie right on top of each other (no shift, no twist).

FIG. 23c illustrates AA′-stacking, which is similar to AA-stacking, butin which the crystallographic axes of the two layers 104 a, 104 b aretwisted by an angle Θ between 0° and 60° with respect to one another (noshift, twist angle Θ).

By means of twisting the layers by an angle of Θ=60°, an ABconfiguration can be transformed into an AA configuration andvice-versa, as illustrated in FIGS. 24a and 24 b.

In order to control the electrical, thermal, and magnetic properties ofa stacked copper graphene bi-layer composite material, and in particularthe wire 82, the twist angle Θ between the layers 104 a, 104 b may beadjusted by means of a die unit 94 in which the openings 98 a, 98 b, 98c have a non-zero inclination angle with respect to a feeding directionof the spiral 90.

As illustrated in FIG. 22a , a main twist angle Θ may be determined bymeans of the inclination angle of the openings 98 a, 98 b, 98 c of thedie unit 94. Different openings 98 a, 98 b, 98 c may have differentinclination angles with respect to a feeding direction of the spiral 90,which may lead to modifications and fine-tuning of the twist anglebetween the individual layers of the spiral 90.

The description of the embodiments and the Figures merely serve toillustrate the techniques of the disclosure, but should not beunderstood to imply any limitation. The scope is to be determined on thebasis of the appended claims.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a” and “an” and “the” and “at least one” andsimilar referents in the context of describing the invention (especiallyin the context of the following claims) are to be construed to coverboth the singular and the plural, unless otherwise indicated herein orclearly contradicted by context. The use of the term “at least one”followed by a list of one or more items (for example, “at least one of Aand B”) is to be construed to mean one item selected from the listeditems (A or B) or any combination of two or more of the listed items (Aand B), unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

What is claimed is:
 1. A wire-drawing method, comprising: providing arod comprising a wrapped sheet, wherein the sheet comprises a pluralityof copper layers and a plurality of graphene layers; extracting an innerlayer of the wrapped sheet from the rod to form a spiral; and forming awire by feeding the spiral through an opening of a die unit.
 2. Themethod according to claim 1, wherein the opening has at least one of anelliptic cross section and a circular cross section.
 3. The methodaccording to claim 1, wherein the opening has an inclination angle withrespect to a feeding direction of the spiral.
 4. The method according toclaim 3, wherein the inclination angle is larger than 5°.
 5. The methodaccording to claim 3, wherein the inclination angle is smaller than 30°.6. The method according to claim 1, wherein forming the wire comprises:feeding the spiral through a first opening of the die unit, andsubsequently feeding the spiral through a second opening of the die unitdownstream of the first opening; wherein the second opening is smallerthan the first opening.
 7. The method according to claim 6, wherein across section of at least one of the first opening and the secondopening has one of an elliptic cross section and a circular crosssection.
 8. The method according to claim 6, wherein the first openinghas a first inclination angle with respect to a feeding direction of thespiral, and wherein the second opening has a second inclination anglewith respect to a feeding direction of the spiral.
 9. The methodaccording to claim 8, wherein the first inclination angle or the secondinclination angle is larger than 5°.
 10. The method according to claim8, wherein the first inclination angle or the second inclination angleis smaller than 30°.
 11. The method according to claim 8, wherein thesecond inclination angle differs from the first inclination angle. 12.The method according to claim 1, wherein feeding the spiral comprisespulling and/or pushing the spiral through the opening.
 13. The methodaccording to claim 12, wherein feeding the spiral comprises pulling thespiral through the opening by means of a plurality of rotating pullingrolls.
 14. The method according to claim 1, wherein the sheet comprisesa first copper layer on a first surface side and a second copper layeron a second surface side opposite from the first surface side.
 15. Themethod according to claim 1, wherein the sheet comprises a copper layerand first and second graphene layers sandwiching the copper layer.
 16. Awire-drawing system, comprising: a die unit comprising an opening; anextracting unit configured to and operating to extract an inner layer ofa wrapped sheet to form a spiral, wherein the sheet comprises aplurality of copper layers and a plurality of graphene layers; and afeeding unit arranged to feed the spiral through the opening of the dieunit.
 17. The system according to claim 16, wherein the opening has oneof an elliptic cross section and a circular cross section.
 18. Thesystem according to claim 16, wherein the opening has an inclinationangle with respect to a feeding direction of the spiral.
 19. The systemaccording to claim 16, wherein die unit comprises a first opening and asecond opening downstream of the first opening in a feeding direction,wherein the second opening is smaller than the first opening.
 20. Thesystem according to claim 16, wherein the feeding unit comprises apulling unit adapted to pull the spiral through the opening.